Method of fabricating a semiconductor device with amorphous carbon layer

ABSTRACT

The invention provides a semiconductor device in which interlayer insulative layers are composed of amorphous carbon film. The amorphous carbon film may include fluorine (F) therein. The invention further provides a method of fabricating a semiconductor device including an interlayer insulative layer composed of amorphous carbon film including fluorine (F), the method having the step of carrying out plasma-enhanced chemical vapor deposition (PCVD) using a mixture gas including (a) at least one of CF 4 , C 2  F 6 , C 3  F 8 , C 4  F 8  and CHF 3 , and (b) at least one of N 2 , NO, NO 2 , NH 3  and NF 3 . The method provides amorphous carbon film having superior heat resistance and etching characteristics. By composing interlayer insulative layers of a semiconductor device of the amorphous carbon film, the semiconductor device can operate at higher speed.

This is a divisional of application Ser. No. 08/526,902 filed Sep. 12,1995.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a semiconductor device, and more particularlyto a semiconductor device including interlayer insulative layerscomposed of insulative material having a low dielectric constant tothereby reduce wire delay and hence make it possible for a semiconductordevice to operate at higher speed than a conventional semiconductordevice. The invention also relates to a method of fabricating such asemiconductor device.

2. Description of the Related Art

With the demand of decreasing wiring width and a spacing between wiringsin a semiconductor device, wire floating capacitance and wire resistancein a semiconductor device are increased with the result of increase ofwire delay which is an obstacle to higher speed operation of asemiconductor device. Thus, for the purpose of decreasing wire delay, itis presently attempted to improve insulative material to be used formultiple wiring layers. Since wire delay is in general in proportion toa root of a dielectric constant of the insulative material, wire delaycan be decreased by composing multiple wiring layers of insulativematerial having a low dielectric constant. Interlayer insulative filmsare presently composed of SiO₂ and the like which has a dielectricconstant of about 4, and it is now desired to develop insulativematerial having a dielectric constant of 3 or smaller. For this purpose,an attempt is being made to provide insulative material having a lowdielectric constant such as SiOF film composed of SiO₂ containingfluorine (F) to thereby reduce a dielectric constant, and organicmaterial such as polyimide having a smaller dielectric constant thanthat of inorganic material such as SiO₂.

For instance, the inventor had published a paper on plasma fluorinationof polyimide in 55th Meeting of Applied Physics Institution, No. 3.21a-G-11, Sep. 19, 1994. The fluorinated polyimide has a dielectricconstant of 3 or smaller. It should be noted that the applicant does notadmit the article No. 3. 21a-G-11 as prior art. This article is citedherein solely for better understanding of the background of theinvention.

For another instance, Japanese Unexamined Patent Public Disclosure No.4-174912 laid open on Jun. 23, 1992 has suggested a cable comprising alinear conductor having a diameter of 1 mm or less, and a plasmapolymerized insulative film covering around the conductor, which filmhas a dielectric constant of 3 or smaller.

Still another instance is an article entitled "Mechanisms of etching andpolymerization in radiofrequency discharges of CF₄ --H₂, CF₄ --C₂ F₄, C₂F₆ --H₂, C₃ F₈ --H₂ " reported by R. d'Agostino, F. Cramarossa, V.Colaprico, and R. d'Ettole through American Institute of Physics in J.Appl. Phys. 54(3), pp 1284-1288, March 1983. This report has reportedsome results obtained during the etching of Si or the deposition offluorocarbon films over Si substrates uncoupled from ground in rfplasmas fed with CF₄ --H₂, C₂ F₆ --H₂, C₃ F₈ --H₂ and CF₄ --C₂ F₄mixtures.

Yet another instance is an article entitled "Electrical and StructuralStudies of Plasma-polymerized Fluorocarbon Films" reported by N. Amyot,J. E. Klemberg-Sapieha, and M. R. Wertheimer in IEEE Transactions onElectrical Insulation, Vol. 27 No. 6, pp 1101-1107, December 1992. Inthis study, plasma-polymerized fluorocarbon films up to 8 μm inthickness have been prepared by high frequency glow discharge depositionto investigate the material's charge storage (electret) properties.Under `mild` plasma conditions, materials with high fluorineconcentration (F/C<1.9) could be obtained, while films with lower F/Cwere found to be partially oxidized.

Still yet another instance is an article entitled "Plasma-depositedamorphous carbon films as planarization layers" reported by S. W. Pangand M. W. Horn through American Vacuum Society in J. Vac. Sci. Technol.B8(6), pp 1980-1984, November/December 1990. According to the report, adry planarization process was developed that utilizes plasma-enhancedchemical vapor deposition of amorphous carbon films. The characteristicsof the films depend on deposition conditions such as source gascomposition, rf power, degree of ion bombardment, temperature, pressure,and electrode spacing. Planar films were deposited at low temperatures(<50° C.) with low ion bombardment energy (<10 V) and high depositionrates (100-300 nm/min).

However, SiO₂ containing fluorine therein does not exhibit sufficientdecrease of a dielectric constant, and merely exhibits a dielectricconstant of about 3. In addition, an interlayer insulative film composedof SiO₂ containing fluorine has a problem with respect to hygroscopicproperty thereof. On the other hand, an interlayer insulative filmcomposed of the polyimide resin also has problems that such a film has alow upper limitation with respect to heat resistance, specifically, theheat resistance of the film is just about 400 degrees centigrade, andthat humidity present in the film exerts a bad influence on asemiconductor device in wet processes, and further that volumetricshrinkage which occurs while the polyimide resin is being cured maycauses the film to be cracked.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a semiconductordevice including interlayer insulative layers composed of insulativematerial having a low dielectric constant.

Another object of the present invention is to provide a method offabricating the above mentioned semiconductor device.

The invention provides a semiconductor device in which interlayerinsulative layers are composed of amorphous carbon film. This amorphouscarbon film is insulative material which exhibits a dielectric constantof 3 or smaller even if it contains no fluorine (F).

The amorphous carbon film may include fluorine (F) therein. By additionof fluorine to the amorphous carbon film, it is possible to reduce adielectric constant down to 2.5 or smaller. The amorphous carbon filmhas a structure in which carbon atoms are cross-linked in high degree.This structure ensures higher heat resistance than polyimide and nohumidity to occur in the amorphous carbon film during polymerization.

It is possible to decrease wire delay without degrading reliability of asemiconductor device by composing interlayer insulative layers of asemiconductor device of the amorphous carbon film.

The amorphous carbon film is formed by making monomer molecules ofhydrocarbon into plasmatic condition to thereby producing radicalmolecules and ions of carbon, and activating such carbon radicalmolecules and ions on a semiconductor substrate. Monomer molecules to beused include hydrocarbon family gas such as CH₄, C₂ H₄ and C₂ H₂, andnaphthalene molecules in solid or liquid phase.

In order to fluorinate the amorphous carbon film, fluorine family gassuch as CF₄, C₂ F₈, C₂ F₄, C₂ F₂ and SF₆ are also used together with theabove mentioned hydrocarbon family gas. From these fluorine family gasare produced fluorine radicals and ions through plasma to thereby addfluorine (F) into the amorphous carbon film.

If there exists fluorine (F) on an interface between the amorphouscarbon film and an underlying layer disposed below the amorphous carbonfilm, cohesion of the amorphous carbon film with the underlying layer isdeteriorated with the result that the amorphous carbon film is prone tobe peeled off. Thus, it is preferable that the content of fluorine (F)has a distribution in a depthwise direction of the amorphous carbonfilm. More specifically, it is preferable that the distribution isdesigned so that no fluorine (F) is present at an interface between theamorphous carbon film and the underlying layer disposed below theamorphous carbon film.

FIGS. 1A and 1B illustrates a bipolar transistor and a MOS type fieldeffect transistor (MOSFET), respectively, in each of which an interlayerinsulative layer is composed of the fluorinated amorphous carbon film.

The bipolar transistor illustrated in FIG. 1A has a p type semiconductorsubstrate 11 in which an n⁺ type diffusion layer 9 is formed. On the n⁺type diffusion layer 9 is formed an n type layer 8 by epitaxy, andadjacent to the n type layer 8 is formed a p⁺ type isolation layer 10 byion implantation. On the epitaxial layer 8 is formed a p type layer 7serving as a base, and adjacent to the p type layer 7 is formed an n⁺type emitter layer 6. An n⁺ type layer 5 is formed for connecting the n⁺type diffusion layer 9 with an n⁺ type polysilicon electrode 4 servingas a collector. On the n⁺ type emitter layer 6 is formed a metalelectrode 2 serving as a gate. The amorphous carbon film or fluorinatedamorphous carbon film is used as an interlayer insulative layer forcovering active regions and wiring electrodes in the bipolar transistor.

A semiconductor device constructed as MOSFET illustrated in FIG. 1B hasa p type semiconductor substrate 11 on which field SiO₂ oxidation films16 are formed except areas which would be used as active regions. In theactive regions are formed a source 14 and a drain 15 by ionimplantation. Centrally between the source 14 and the drain 15 is formeda gate electrode 13 on a thin SiO₂ film (not illustrated), whichelectrode 13 is composed of polysilicon. The amorphous carbon film isdeposited so that it fully covers these contacts.

The above mentioned amorphous carbon film containing fluorine thereinexhibits a dielectric constant of about 2.1. Though this amorphouscarbon film has a sufficiently low dielectric constant, it has smallerheat resistance temperature than SiO₂, resulting in that the fluorinatedamorphous carbon film can have only limited range of uses. For instance,the amorphous carbon film containing fluorine therein begins to bethermally decomposed at about 420 degrees centigrade with the result ofdecrease in film thickness and gas generation. Thus, it is necessary tokeep heat treatment temperature below 420 degrees centigrade when theamorphous carbon film is to be used. However, a semiconductor devicefabrication process often needs heat treatment at high temperature, andhence it is necessary to modify the amorphous carbon film so that it canwithstand heat treatment at temperature of at smallest 450 degreescentigrade.

This can be accomplished by introducing another atoms into a fluorinatedamorphous carbon film. Since the fluorinated amorphous carbon film isformed by using carbon fluoride family gas or a mixture gas of fluorinefamily gas and hydrogen gas, the fluorinated amorphous carbon film ingeneral contains carbon, fluorine and hydrogen atoms. The carbon atomsmake carbon-carbon bonds in the film to thereby form a core of the film.The fluorine atoms decrease a dielectric constant of the film. Thehydrogen atoms have a function of terminating non-bonding orbits in thefilm. By introducing nitrogen atoms or silicon atoms into the amorphouscarbon film, there are produced strong bonds such as carbon-nitrogen andcarbon-silicon in the amorphous carbon film to thereby increasecross-linking degree of the film, which in turn enhances heat-resistanceand etching resistance of the film.

Thus, the invention further provides the amorphous carbon film includingfluorine (F) and further nitrogen (N). The amorphous carbon film mayinclude silicon (Si) in place of nitrogen.

The invention still further provides a semiconductor device in whichinterlayer insulative layers are composed of amorphous carbon film andwhich includes a buffer layer for suppressing gas discharge out of theamorphous carbon film, the buffer layer disposed between the amorphouscarbon film and elements of the semiconductor device disposed adjacentto the amorphous carbon film. Herein, the elements of the semiconductordevice means, for instance, an electrode, a wiring and a transistorsection.

In a preferred embodiment, the semiconductor device further includes atransition layer interposed between the amorphous carbon film and thebuffer layer, the transition layer having a composition graduallyvarying from a composition of the amorphous carbon film to a compositionof the buffer layer.

In another preferred embodiment, the buffer layer has a smallerthickness than that of the amorphous carbon film. The thickness of thebuffer layer is determined in accordance with temperature of heattreatment to be carried out in fabrication of the semiconductor device.

Thus, a semiconductor device in accordance with an embodiment of theinvention includes a buffer layer interposed between the amorphouscarbon film and elements such as electrodes and wirings. The bufferlayer suppresses gas discharge out of the amorphous carbon film in heattreatment process, and hence provides a semiconductor device which canwithstand heat treatment at higher temperature. Material of which thebuffer layer is composed may be selected from SiO2 , Si3N4, amorphouscarbon film including silicon therein, or amorphous carbon filmincluding nitrogen therein.

It should be noted that material of which the buffer layer is composedis not limited to the above mentioned ones. Any material may be selectedif it is highly densified so that it can interrupt gas discharged out ofan amorphous carbon film from breaking out therethrough. In general,such material has a greater dielectric constant than that of anamorphous carbon film. For instance, SiO₂, Si₃ N₄, and an amorphouscarbon film containing nitrogen or silicon have a dielectric constant of4, 7 and 3, respectively. However, as aforementioned, by forming thebuffer layer in a smaller thickness than that of the amorphous carbonfilm, it is possible to reduce a dielectric constant of the amorphouscarbon film containing the buffer layer down to a dielectric constant ofan amorphous carbon film containing no buffer layer.

The invention still further provides a method of fabricating amorphouscarbon film including fluorine (F), of which an interlayer insulativelayer of a semiconductor device is to be composed, which methodincluding the step of carrying out plasma-enhanced chemical vapordeposition (PCVD) using CxFy gas, wherein x is an integer ranging from 1to 4 both inclusive, and y is an integer ranging from 4 to 8 bothinclusive, so that a substrate on which the amorphous carbon film is tobe deposited is disposed outside an area in which plasma is generated.

The invention yet further provides a method of fabricating amorphouscarbon film including fluorine (F), of which an interlayer insulativelayer of a semiconductor device is to be composed, the method includingthe step of carrying out plasma-enhanced chemical vapor deposition(PCVD) using CxFy gas, wherein x is an integer ranging from 1 to 4 bothinclusive, and y is an integer ranging from 4 to 8 both inclusive, sothat a substrate on which the amorphous carbon film is to be depositedis disposed outside an area in which plasma is generated, and providinghigh-frequency electric power with the substrate while PCVD is beingcarried out.

The reason why the above mentioned methods are provided is as follows.Interlayer insulative material for isolating wirings from one another isrequired to have a dielectric constant which is reduced possibly to thesmallest, an ability with which the material sufficiently fills spacingbetween patterned wirings, and deposition speed of at least about 0.1μm/min in order to increase fabrication efficiency per unit time. Thepresently used SiO₂ interlayer insulative layer can satisfy therequirements with respect to space-filling property and deposition speedamong the properties required to the insulative material, by usinghighly densified plasma and further applying a bias voltage to asemiconductor substrate. However, the SiO₂ interlayer insulative layercan merely have a dielectric constant of about 4, and it is quitedifficult to decrease a dielectric constant to the range smaller than 4.If a fluorinated amorphous carbon film is to be used as anotherinsulative material, it is possible to decrease a dielectric constant tothe range of 3 or smaller, but an interlayer insulative layer composedof a fluorinated amorphous carbon film is inferior to the SiO₂ layerwith respect to the film deposition speed and planarization obtainedafter spacings of patterned wirings are filled with the film.

The reason why a conventional fluorinated amorphous carbon film hasslower film deposition speed and inferior planarization of patternedwirings is that since film deposition is performed by low densifiedplasma, monomer as raw material is decomposed through plasma in smallerspeed, and hence density of fluorinated carbon radicals contributing tofilm deposition is small. For this reason, it takes more than 30 minutesto deposit a 1 μm thickness film. Thus, it is necessary to make thedeposition speed to be two times or more greater for practical use. Inaddition, conventional parallel flat plate type low densified plasmacannot deposit an amorphous carbon film only by using carbon fluoridegas. It was necessary to add hydrogen gas when a film is to bedeposited. The added hydrogen atoms make a bond with carbon atoms, andthus become a part of the film. The hydrogen atoms present in the filmdegrade the cross-linking degree of the film with the result ofdeterioration of heat resistance of the film. Accordingly, it is nowdesired to develop a process in which film deposition can be performedwithout addition of hydrogen. Film structure of an amorphous carbon filmis remarkably changed by ion irradiation. In a conventional parallelflat plate type low densified plasma, since irradiated ions have largeamounts of energy because of self-bias to be applied to a substrate, itis difficult in principle to control ion energy so that the ion energyis kept in an optimal value for amorphous carbon film deposition,resulting in deterioration of planarization of patterned wirings.

The problem as mentioned above is overcome with the methods inaccordance with the invention. In the method in accordance with theinvention, highly densified plasma is used in order to increase radicaldensity which contributes to film deposition speed. In addition, since asemiconductor substrate is disposed outside an area in which plasma isgenerated, it is possible to decrease ion energy to be irradiated onto asemiconductor substrate, and hence it is also possible to deposit afluorinated amorphous carbon film without addition of hydrogen.Furthermore, in the method in accordance with the invention, ahigh-frequency voltage is applied to a semiconductor substrate on whichthe amorphous carbon film is to be deposited, and thus a voltage of thesubstrate is optimized, thereby making it possible to vary ion energyand control film quality such as heat resistance and planarization.

The fluorinated amorphous carbon film to be fabricated in accordancewith the invention is formed by exciting fluorinated carbon familymonomer molecules through plasma, such as CF₄, C₂ F₆, C₃ F₈ and C₄ F₈,and activating the thus generated radical molecules and ions on asemiconductor substrate. Film deposition through plasma is in generalconsidered to occur due to a combination of deposition reaction ofradicals having deposition characteristic, and etching reaction causedby ions or radicals having etching property. If highly densified plasmais used as plasma source, density of fluorinated carbon radicals havingdeposition property is increased, since decomposition speed of monomermolecules are increased relative to a parallel flat plate type plasmagenerator. Hence, film deposition is facilitated with the result ofhigher film deposition speed.

In a conventional parallel flat plate type plasma, since ions areaccelerated by self-bias voltage applied to a semiconductor substrate,etching carried out by ion irradiation is facilitated. Accordingly, ifplasma is generated only with CF family gas, etching speed becomesgreater than film deposition speed, and hence deposition of fluorinatedamorphous carbon film can not occur. In order to deposit the amorphouscarbon film, fluorine atoms considered to serve as an etcher have to beremoved by adding hydrogen gas and the like.

In the method in accordance with the invention, highly densified plasmasuch as helicon wave discharge and microwave discharge is used, and inaddition, an area in which plasma is to be produced is separate from anarea in which film deposition is performed. This makes it possible tomake ion energy small regardless of high ion density. In these highlydensified plasma sources, etching is suppressed, and thus filmdeposition can be performed without addition of hydrogen gas. Hence, itis now possible to remove hydrogen atoms which have been present in anamorphous carbon film in conventional parallel flat plate type plasmaand have degraded heat resistance of the amorphous carbon film. Inaddition, since a voltage of a substrate is controlled by applying ahigh-frequency electric power thereto, irradiation ion energy isoptimized with the result of enhancement of heat resistance andplanarization of the film.

The invention further provides a method of fabricating amorphous carbonfilm including fluorine (F), of which an interlayer insulative layer ofa semiconductor device is to be composed, which method including thestep of carrying out plasma-enhanced chemical vapor deposition (PCVD)using a mixture gas including (a) at least one of CF₄, C₂ F₆, C₃ F₈, C₄F₈ and CHF₃, and (b) at least one of N₂, NO, NO₂, NH₃ and NF₃.

In a preferred embodiment, the mixture gas further includes (c) at leastone of H₂, CH₄, C₂ H₆, C₂ H₄, C₂ H₂, and C₃ H₈.

The invention still further provides a method of fabricating amorphouscarbon film including fluorine (F), of which an interlayer insulativelayer of a semiconductor device is to be composed, which methodcomprising the step of carrying out plasma-enhanced chemical vapordeposition (PCVD) using a mixture gas including (a) at least one of CF₄,C₂ F₆, C₃ F₈, C₄ F₈ and CHF₃, and (b) least one of SiH₄, SiH₆, and SiF₄.

In a preferred embodiment, the mixture gas further includes (c) at leastone of H₂, CH₄, C₂ H₆, C₂ H₄, C₂ H₂, and C₃ H₈.

The above mentioned amorphous carbon film containing fluorine thereinexhibits a dielectric constant of about 2.1. Though this amorphouscarbon film has a sufficiently low dielectric constant, it has smallerheat resistance temperature than SiO₂, resulting in that it can haveonly limited range of use. For instance, the amorphous carbon filmcontaining fluorine therein begins to be thermally decomposed at about420 degrees centigrade with the result of decrease in film thickness andgas generation. Thus, it is necessary to keep heat treatment temperaturebelow 420 degrees centigrade when the amorphous carbon film is to beused.

When the amorphous carbon film is to be used for interlayer insulativematerial, it is necessary to carry out patterning by means ofconventional lithography. However, since the amorphous carbon materialconsists mainly of carbon similarly to resist used in lithography, evenif it is etched with CF4 or CHF3 gas, it is impossible to make aselection ratio between etching and resist greater. For instance, whenan amorphous carbon film having a thickness of 1 μm is to be patterned,resist has to be covered over the amorphous carbon film by about 2 μm orgreater. In addition, when resist is to be removed, the resist is ashedgenerally by oxygen plasma. However, since the amorphous carbon film isalso ashed together with the amorphous carbon film, the amorphous carbonfilm has to have a structure which is difficult to be etched by oxygenplasma.

Such a structure is accomplished by introducing another atoms intofluorinated amorphous carbon film. Since the fluorinated amorphouscarbon film is formed by using carbon fluoride family gas or a mixturegas of fluorine family gas and hydrogen gas, the fluorinated amorphouscarbon film in general contains carbon, fluorine and hydrogen atoms. Thecarbon atoms make carbon-carbon bonds in the film to thereby form a coreof the film. The fluorine atoms decrease a dielectric constant of thefilm. The hydrogen atoms have a function of terminating non-bondingorbits in the film. By introducing nitrogen atoms or silicon atoms intothe amorphous carbon film, there are produced strong bonds such ascarbon-nitrogen and carbon-silicon in the amorphous carbon film tothereby increase cross-linking degree of the film, which in turnenhances heat-resistance and etching resistance of the film.

The factor which determines heat-resistance of a film composed of carbonis a cross-linking structure of the film. Herein, the cross-linkingstructure means a structure in which carbon-carbon bonds exist in randomin a film in question. Conventional fluororesin has a structurerepresented by a formula (CF₂)n, that is, a structure in whichcarbon-carbon bonds extend like a chain. In such a structure, chain-likemolecules are bonded by Van der Waals forces, and thus fluororesin doesnot have a cross-linking structure. For this reason, fluororesin beginsits thermal decomposition at 300 degrees centigrade. Thus, fluororesinhas a low heat-resistance. However, since an amorphous carbon film is ingeneral deposited by dissociating hydrogen fluoride family gas byplasma, carbon-carbon bonds distribute in random in the film. Thus, theamorphous carbon film has a cross-linking structure, and hence can havegreater heat-resistance than that of fluororesin. Specifically,components of the film begins to be desorbed at about 420 degreescentigrade. It is considered that the desorption of the film componentsout of the fluorinated amorphous carbon film occurs because side chains,which are present in the film, such as --CF₃ or --(CF₂)n--CF₃ are madebroken at about 420 degrees centigrade.

If these side chains can be bundled with new bonds to thereby increasethe cross-linking degree, it is possible to raise desorptiontemperature. In the method in accordance with the invention, other atomsare introduced into the film to bundle side chains to thereby increasethe cross-linking degree. Any atoms may be selected for increasing thecross-linking degree, if they can be supplied in gas phase, they canform covalent bonds with carbon atoms, and a resultant containing themcan maintain insulating property. In the method in accordance with theinvention, either nitrogen atoms having three configuration or siliconatoms having four configuration is added into the film so that theseatoms make bonds with carbon atoms to newly generate a cross-linkingstructure in a side-chain having a low degree of cross-linking. Inaddition, it is possible to make oxygen plasma etching speed smallerthan that of an ordinary amorphous carbon film by utilizing the factthat carbon-silicon bond and carbon-nitrogen bond have a greater bondingforce than that of carbon-carbon bond. Thus, even if resist is ashed,the amorphous carbon film would not be ashed. Furthermore, when anamorphous carbon film is to be etched with carbon fluoride family gas inpatterning, it is possible to raise etching speed up to SiO₂ etchingspeed by adding silicon into the film, resulting in that patterningsteps which are the same as those for patterning SiO₂ are able to beused.

The advantages obtained by the aforementioned present invention will bedescribed hereinbelow.

As having been described, it is now possible to make a semiconductordevice operate at higher speed without deteriorating reliability bycomposing an interlayer insulative layer of an amorphous carbon film, anamorphous carbon film containing fluorine (F) therein, or an amorphouscarbon film containing fluorine and nitrogen (N) or silicon (Si).

Furthermore, by forming a thin buffer layer between the amorphous carbonfilm and elements such as electrodes and wirings for suppressing gasdischarge out of the amorphous carbon film which is to occur when theamorphous carbon film is subject to heat treatment, it is possible toprevent degradation of electrodes, wirings, etc. due to the gasdischarge from the amorphous carbon film, and hence also possible toprovide a semiconductor device having high heat resistance.

The amorphous carbon film fabricated in accordance with the method ofthe present invention has superior heat resistance and etchingcharacteristics. Thus, by composing an interlayer insulative film of asemiconductor device of the amorphous carbon film, it is possible tomake a semiconductor device operate at higher speed withoutdeteriorating reliability.

In addition, the method, in which a substrate on which the amorphouscarbon film is to be deposited is disposed outside an area in whichplasma is generated, makes it possible to form a fluorinated amorphouscarbon film containing no hydrogen. Furthermore, by providinghigh-frequency electric power with the substrate while PCVD is beingcarried out, it is possible to optimize ion energy and hence form anamorphous carbon film having high heat resistance.

The above and other objects and advantageous features of the presentinvention will be made apparent from the following description made withreference to the accompanying drawings, in which like referencecharacters designate the same or similar parts throughout the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional view of a semiconductor device constructedof a bipolar transistor having an interlayer insulative layer composedof the amorphous carbon film;

FIG. 1B is a cross-sectional view of a semiconductor device constructedof MOSFET having an interlayer insulative layer composed of theamorphous carbon film;

FIG. 2 is a schematic view illustrating an apparatus for depositing thefluorinated amorphous carbon film containing nitrogen or silicon;

FIG. 3 is a cross-sectional view of a semiconductor device having aninterlayer insulative layer composed of the amorphous carbon film;

FIGS. 4A and 4B are graphs showing distribution of fluorine content in adepthwise direction of the amorphous carbon film;

FIG. 5 is a graph showing a relationship between fluorine content of theamorphous carbon film and CF₄ /CH₄ flow ratio;

FIG. 6 is a graph showing distribution of fluorine content in adepthwise direction of the fluorinated amorphous carbon film;

FIG. 7 is a graph showing current-voltage characteristic of theamorphous carbon film and the fluorinated amorphous carbon film;

FIG. 8 is a graph showing a relationship between signal delay time andconcentration of fluorine molecules in the semiconductor device inaccordance with the invention;

FIG. 9 is a graph showing a relationship between nitrogen content in theamorphous carbon film and a flow ratio of N₂ gas to total gas;

FIG. 10 is a graph showing how heat resistance of the amorphous carbonfilm varies in dependence on a flow ratio of N₂ gas to total gas;

FIG. 11 is a graph showing a relationship between a dielectric constantof the amorphous carbon film and nitrogen content of the film;

FIG. 12 is a graph showing a relationship between silicon content and aflow ratio of SiH₄ to total gas;

FIG. 13 is a graph showing how heat resistance of the amorphous carbonfilm varies in dependence on a flow ratio of SiH₄ gas to total gas;

FIG. 14 is a graph showing a relationship between a dielectric constantof the amorphous carbon film and silicon content of the film;

FIG. 15 is a graph showing etching speed of the amorphous carbon filmdeposited under O₂ plasma;

FIG. 16 is a graph showing etching speed of the amorphous carbon filmdeposited under CF₄ plasma;

FIG. 17A is a cross-sectional view of a semiconductor device constructedof a bipolar transistor having an interlayer insulative layer composedof the amorphous carbon film and the buffer layer interposed between theinterlayer insulative layer and other components;

FIG. 17B is a cross-sectional view of a semiconductor device constructedof MOSFET having an interlayer insulative layer composed of theamorphous carbon film and the buffer layer interposed between theinterlayer insulative layer and other components;

FIG. 18A shows a structure of a semiconductor device having thetransition layer between the amorphous carbon film and the buffer layer;and

FIG. 18B shows a structure of a semiconductor device having notransition layer.

FIG. 19 is a schematic view of an apparatus for depositing a fluorinatedamorphous carbon film through the use of helicon wave plasma;

FIG. 20 is a schematic view of an apparatus for depositing a fluorinatedamorphous carbon film through the use of microwave plasma;

FIG. 21 is a graph showing deposition speed of a fluorinated amorphouscarbon film deposited by using a parallel flat plate type plasma source;

FIG. 22 is a graph showing a dielectric constant of a fluorinatedamorphous carbon film deposited by using a parallel flat plate typeplasma source;

FIG. 23 is a graph showing the dependency of deposition speed on H₂ flowrate when helicon wave plasma source is used;

FIG. 24 is a graph showing the dependency of deposition speed on H₂ flowrate when microwave plasma source is used; and

FIG. 25 is a graph showing the dependency of deposition speed on a biaselectric power.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Preferred embodiments in accordance with the present invention will beexplained hereinbelow with reference to drawings.

FIG. 2 illustrates an apparatus for forming an amorphous carbon filmcontaining fluorine therein. The apparatus includes a support plate 1 onwhich is formed a vacuum chamber 22 having a top cover 23 forhermetically sealing of the chamber 22. In the vacuum chamber 22 aredisposed an upper electrode 24 and a lower electrode 25 in facingrelation to each other. A high frequency voltage supply 26 applies dc orac electric power across the electrodes 24 and 25. On the lowerelectrode 25 is placed a specimen 27. The lower electrode 25 is inthermal communication with a heater 28 for heating the specimen 27 to adesired temperature. The vacuum chamber 22 is in communication with avacuum pump 29 so that the vacuum chamber 22 is made vacuous. A gascontainer 30 supplies hydrocarbon gas into the vacuum chamber 22.

For forming an amorphous carbon film, a specimen 27 such as a siliconsubstrate is placed either on the lower electrode 25 or on the upperelectrode 24. The high frequency power applied to the lower electrode 25ensures that a few hundreds of negative bias voltage is applied to thelower electrode 25. When the specimen 27 is placed on the lowerelectrode 25 for film deposition, ion accelerated by the bias areirradiated over the specimen 27, and there is obtained an amorphouscarbon film which contains less amount of hydrogen therein and has across-linked structure in greater degree. On the other hand, when thespecimen 27 is placed on the grounded upper electrode 24, ions are notaccelerated by the lower electrode 25, and hence there is obtained anamorphous carbon film containing larger amount of hydrogen.

After the specimen 27 has been placed on one of the electrodes 24 and25, hydrocarbon gas such as CH₄, C₂ H₄, and C₂ H₂ is introduced into thevacuum chamber 22 from the gas container 30, and then high frequencyelectric power or dc electric power is applied across the electrodes 24and 25 at 0.01-0.5 Torr to thereby cause glow discharge. As a result,there is generated hydrocarbon plasma.

The thus generated hydrocarbon plasma makes an amorphous carbon filmdeposit on the specimen 27. Prior to or during deposition of theamorphous carbon film, the specimen 27 is heated by the heater 28 to adesired temperature in order to control reaction of radicals and ionsand hence film quality such as hydrogen content of the amorphous carbonfilm.

When a fluorinated amorphous carbon film is to be deposited, fluorinefamily gas such as CF₄, SF₆, C₂ F₄, NF₃, C₂ F₆, C₃ F₈ and C₄ F₈ isconcurrently introduced together with the above mentioned hydrocarbongas into the vacuum chamber 22 from the gas container 30. Subsequentsteps are the same as those for depositing the amorphous carbon film.

Hereinbelow will be explained a detailed experimental example. FIG. 3shows a cross-section of a semiconductor device having an interlayerinsulative layer composed of the amorphous carbon film in accordancewith the invention.

First, a transistor was fabricated on a silicon substrate 31 in aconventional manner. On the silicon substrate 31 was selectivelydeposited field SiO₂ films 32 to define active regions. After materialsuch as aluminum for formation of an electrode was deposited, wiringswere patterned by means of conventional lithography technique to therebyform a first aluminum layer 33. Then, the silicon substrate 31 on whichthe aluminum wiring 33 had been formed was placed in the vacuum chamber22 of the apparatus illustrated in FIG. 2.

Into the vacuum chamber 22 was introduced hydrocarbon gas such as CH₄,C₂ H₄, and C₂ H₂, and then high frequency electric power was appliedacross the electrodes 24 and 25 to make a discharge to thereby developthe hydrocarbon gas into plasmatic condition. In place of thehydrocarbon gas, there may be used solid material such as naphthalene orhydrocarbon in liquid phase. In order to make a discharge, there may beused, in place of the above mentioned high frequency discharge, directcurrent discharge, microwave discharge, magnetron type discharge, andinductive coupling type discharge in which a coil is used for making adischarge. The thus generated hydrocarbon radical molecules and ionsmake an amorphous carbon film deposit on the silicon substrate 31. Then,over the amorphous carbon film 34 was patterned a second aluminum layer35.

When a fluorinated amorphous carbon film is to be deposited, fluorinefamily gas such as CF₄, SF₆, C₂ F₄, NF₃ and C₂ F₆ together with thehydrocarbon gas were introduced into the vacuum chamber 22. Then, therewas generated a plasma to thereby deposit a fluorinated amorphous carbonfilm on the silicon substrate 31.

When only 10 sccm of CH₄ gas was introduced into the vacuum chamber 22and high frequency electric power by 50 W was applied across theelectrodes 24 and 25 at 0.1 Torr at high temperature, resultantamorphous carbon film, which was deposited on the lower electrode 25,had a dielectric constant of 2.9. When high frequency electric power by100 W was applied across the electrodes 24 and 25 under otherwiseunchanged conditions, a dielectric constant of the resultant amorphouscarbon film was raised to 3.2. It is considered that as high frequencyelectric power is increased, an amorphous carbon film is cross-linked ingreater degree with the result of a higher dielectric constant.

Next will be explained an example of a fluorinated amorphous carbonfilm. After a specimen was placed on the lower electrode, 5 sccm of CH₄gas and 50 sccm of CF₄ gas were introduced into the vacuum chamber 22 ofthe plasma generator, and high frequency electric power of 100 W wasapplied across the electrodes. A dielectric constant of the resultantfluorinated amorphous carbon film was decreased down to 2.5.

FIG. 4A shows fluorine content in a depthwise direction of thisfluorinated amorphous carbon film, while FIG. 4B shows fluorine contentof an amorphous carbon film which was deposited by introducing 10 sccmof CH₄ gas into the vacuum chamber 22, and applying high frequencyelectric power of 100 W across the electrodes 24 and 25. The fluorinecontent was measured by secondary ion mass spectrometry. As is obvious,fluorine content of the fluorinated amorphous carbon film (FIG. 4A) istwo figures greater than that of the non-fluorinated amorphous carbonfilm (FIG. 4B). The fluorine content can be controlled by varying flowratio of fluorine family gas to hydrocarbon gas. FIG. 5 shows arelationship between flow ratio of fluorine family gas to hydrocarbongas and fluorine content in the film.

However, if a fluorinated amorphous carbon film is deposited directly onsilicon or aluminum substrate, there is a fear that the depositedamorphous carbon film may be peeled off the substrate because offluorine present in an interface between the film and the substrate. Inorder to avoid peeling off the film, the fluorine content profile isoptimized in the invention as follows.

FIG. 6 shows fluorine content in a depthwise direction of an amorphouscarbon film which was deposited in such a way that fluorine family gaswas not introduced into the vacuum chamber at the initial stage of filmdeposition, and was introduced at the intermediate stage of filmdeposition. By making fluorine content profile in a depthwise directionof the film, it is possible to enhance cohesion of the film with thesubstrate because there exists no fluorine in an interface between theamorphous carbon film and the substrate. In accordance with the resultsof peeling test, a peeling rate of an amorphous carbon film could beimproved by about 80% in comparison with a film which was deposited byintroducing fluorine family gas from the initial stage of filmformation.

In the profile illustrated in FIG. 6, fluorine content graduallydecreases from the amorphous carbon film to the interface. However, itshould be noted that what is necessary is that no fluorine is present atthe interface between the film and the substrate, and hence there may beadopted a steep profile in which fluorine content is sharply decreasedto zero at the interface.

FIG. 7 shows current-voltage characteristic of the fluorinated amorphouscarbon film and a non-fluorinated amorphous carbon film. It isunderstood that the fluorinated amorphous carbon film has enhancedinsulation property relative to the non-fluorinated amorphous carbonfilm. It is considered that this is because trap level present in anamorphous carbon film is terminated by fluorine, and hence there existsno trap level.

FIG. 8 shows a relationship between fluorine content and signal delaytime of a semiconductor device having an interlayer insulative layercomposed of the amorphous carbon film and the amorphous carbon filmcontaining fluorine therein. The graph in FIG. 8 is normalized so thatsignal delay measured in a semiconductor device having an interlayerinsulative layer composed of SiO₂ represents 100%. Even if an interlayerinsulative layer is composed of an amorphous carbon film containing nofluorine is used, the interlayer insulative layer can have a smallerdielectric constant than that of an interlayer insulative layer composedof SiO₂ with the result of higher operation speed of a semiconductordevice. As fluorine content increases, the amorphous carbon film canhave a smaller dielectric constant with the result of decreased signaldelay time. In particular, by composing an interlayer insulative layerof a fluorinated amorphous carbon film having a dielectric constant of2.5, signal delay time could be decreased down to 80%.

The inventor has recognized that even if an amorphous carbon film or afluorinated amorphous carbon film each of which was deposited on theupper electrode is used, higher operation speed of a semiconductordevice can be accomplished. In addition, an amorphous carbon film and afluorinated amorphous carbon film which were deposited using magnetron,helicon wave or microwave could have dielectric constants of 2.9 and2.5, respectively. Thus, it is also possible to accomplish higheroperation speed of a semiconductor device, similarly to the amorphouscarbon film deposited using high frequency discharge, by composing aninterlayer insulative layer of those films.

Referring back to FIG. 2, hereinbelow will be explained a fluorinatedamorphous carbon film containing nitrogen or silicon. First, anembodiment of a fluorinated amorphous carbon film containing nitrogentherein is explained. The fluorinated amorphous carbon film containingnitrogen was deposited using a mixture gas including CF₄, CH₄ and N₂.The deposition was carried out with SiO₂ /Si (100) and P⁺ Si(100)substrates 27 being mounted on the lower electrode 25 to which highfrequency electric power is to be applied. The amorphous carbon film wasdeposited under the condition that the mixture gas flow was kept to beconstant at 50 scam, high frequency electric power was kept to be 200 W,and a CF₄ /CH₄ flow ratio was also kept to be 16, while N₂ gas flow ratewas varied.

A resultant amorphous carbon film was heated to 500 degrees centigradein vacuum. The heat resistance of the film was evaluated with decreasein a film thickness. Nitrogen content of the film was measured from aratio among areas of C1_(S), F1_(S) and N1_(S) peaks of signals obtainedby means of X-ray photoelectron spectrophotometry. A dielectric constantof the amorphous carbon film was measured by measuring a capacity (1MHz) of a capacitor composed of Al/amorphous carbon film/p⁺ Si. FIG. 9shows a relationship between a gas flow ratio of N₂ gas to all the gasand nitrogen content of the film. It is understood that nitrogen contentincreases as the flow ratio increases.

FIG. 10 shows a relationship between heat resistance of the film and thegas flow ratio of N₂ gas to all the gas. Herein, the heat resistance isrepresented by a degree of decrease of a film thickness, namely a ratioof a film thickness measured after heating to a film thickness measuredprior to heating. The film was heated for an hour in vacuum at plottedtemperatures. As is shown in FIGS. 9 and 10, an amorphous carbon filmcan contain nitrogen therein by adding N₂ gas into a process gas, andthe amorphous carbon film containing nitrogen can have enhanced heatresistance relative to an amorphous carbon film containing no nitrogen.In particular, it was found that an amorphous carbon film containingnitrogen by 15% or more can have high heat resistance by which the filmthickness is not reduced even if the amorphous carbon film is heated to470 degrees centigrade.

FIG. 11 shows a relationship between nitrogen content of the amorphouscarbon film and a dielectric constant measured from capacitance of thefilm. The curve 1 shows a dielectric constant of the film prior to heattreatment thereof. As is shown, as the nitrogen content increases, thedielectric constant simply increases. Thus, it has been found that adielectric constant of the film increases with the increase of nitrogencontent thereof, however, the dielectric constant remains smaller than3. The curve 2 shows a dielectric constant of an amorphous carbon filmwhich was subject to heat-treatment for an hour at 300 degreescentigrade in vacuum. An amorphous carbon film containing no or onlysmall amount of nitrogen exhibits raise-up of a dielectric constantunder heat treatment at 300 degrees centigrade, however, it has beenfound that addition of nitrogen into an amorphous carbon film cansuppress such raise-up of a dielectric constant even under 300 degreescentigrade heat treatment. The reason why decrease of a film thicknessand increase of a dielectric constant because of heat treatment do notoccur is considered that C-N bonds are newly formed in the amorphouscarbon film. The bonding energy of C--N is 175 Kcal/mol, while thebonding energy of C--C is 145 Kcal/mol. Thus, the increased heatresistance of the film is considered to be caused by the fact that C--Nbond is more stable than C--C bond.

The inventor observed how nitrogen atoms make a bond with other atoms inthe film by using X-ray photoelectron spectrometry and infraredabsorption spectrometry. The results show that all of nitrogen atoms inthe film exist making C--N bonding, and that N--F bonding do not existin the film. Namely, nitrogen all makes a bond with a carbon atom in theamorphous carbon film to thereby enhance cross-liking degree of thefilm.

Hereinbelow will be explained an example of a fluorinated amorphouscarbon film containing silicon. The fluorinated amorphous carbon filmcontaining silicon was deposited using SiH₄ gas with the high frequencydischarge apparatus illustrated in FIG. 2. SiO₂ /Si (100) and P⁺ Si(100)substrates 27 were mounted on the lower electrode 25 to which highfrequency electric power is to be applied. The amorphous carbon film wasdeposited under the condition that the gas flow was kept to be constantat 50 sccm, high frequency electric power was kept to be 200 W, and aCF₄ /CH₄ flow ratio was also kept to be 16, while Si gas flow rate wasvaried.

A resultant amorphous carbon film was heated to 500 degrees centigradein vacuum. The heat resistance of the film was evaluated with decreasein a film thickness. Silicon content of the film was measured from aratio among areas of C1_(S), F1_(S) and N1_(S) peaks of signals obtainedby means of X-ray photoelectron spectrometry. A dielectric constant ofthe amorphous carbon film was measured by measuring a capacity (1 MHz)of a capacitor composed of Al/amorphous carbon film/p⁺ Si. FIG. 12 showsa relationship between a gas flow ratio of SiH₄ gas to all the gas andsilicon content of the film. It is understood that the amorphous carbonfilm can contain silicon therein only by adding SiH₄ gas into theprocess gas.

FIG. 13 shows heat resistance of the film. Herein, the heat resistanceis represented by a degree of decrease of a film thickness, namely aratio of a film thickness measured after heating to a film thicknessmeasured prior to heating. The film was heated for an hour in vacuum atplotted temperatures. As is shown in FIGS. 12 and 13, an amorphouscarbon film can contain silicon therein by adding silicon gas into aprocess gas, and the amorphous carbon film containing silicon can haveenhanced heat resistance relative to an amorphous carbon film containingno silicon. In particular, it was found that the amorphous carbon filmcontaining silicon by 20% or more can have high heat resistance by whichthe film thickness is not reduced even if the amorphous carbon film isheated to 470 degrees centigrade.

FIG. 14 shows a relationship between silicon content of the amorphouscarbon film and a dielectric constant measured from capacitance of thefilm. The curve 1 shows a dielectric constant of the film measured priorto heat treatment thereof. As is shown, as the silicon contentincreases, the dielectric constant simply increases. Thus, it has beenfound that a dielectric constant of the film increases with the increaseof silicon content thereof, similarly to the amorphous carbon filmcontaining nitrogen, however, the dielectric constant remains smallerthan 3. For instance, the amorphous carbon film containing silicontherein by 20% has a dielectric constant of 2.8. The curve 2 shows adielectric constant of an amorphous carbon film which was subject toheat-treatment for an hour at 300 degrees centigrade in vacuum. Anamorphous carbon film containing small amount of silicon exhibitsraise-up of a dielectric constant by heat treatment to be carried outafter film deposition. However, it has been found that addition ofsilicon into an amorphous carbon film can suppress such raise-up of adielectric constant caused by heat treatment.

The inventor observed how silicon atoms make a bond with other atoms inthe film by using X-ray photoelectron spectrometry and infraredabsorption spectrometry. The results show that all of silicon atoms inthe film exist making Si--C bonding. Thus, it is considered that siliconatoms added into the film make a strong bond with a carbon atom, namelySi--C bonding, to thereby enhance heat resistance of the film.

Etching property of the film was also observed. Into the vacuum chamber22 of the apparatus illustrated in FIG. 2 was supplied 100 sccm of O₂gas, and then an amorphous carbon film was deposited under 200 W of highfrequency electric power. Then, a resultant amorphous carbon film wasetched, and resistance of the film against oxygen plasma was observed.FIG. 15 shows a relationship between etching speed of fluorinatedamorphous carbon film containing nitrogen or silicon when etched byoxygen plasma, and nitrogen or silicon content of the film. By addingnitrogen or silicon into the fluorinated amorphous carbon film, therewas obtained an amorphous carbon film having resistance against oxygenplasma.

Next, etching property when etched with CF₄ gas was observed. Into thevacuum chamber 22 of the apparatus illustrated in FIG. 2 was supplied100 sccm of CF₄ gas, and an amorphous carbon film was deposited under200 W of high frequency electric power. Then, a resultant amorphouscarbon film was etched. FIG. 16 shows etching speed caused by CF₄plasma. With the use of CF₄ gas, etching speed of the fluorinatedamorphous carbon film containing silicon therein was increased relativeto an amorphous carbon film containing no silicon. The reason of this isconsidered as follows. A silicon atom is easier to be etched than acarbon atom in carbon fluoride plasma. Hence, silicon atoms in anamorphous carbon film are first etched, and fluorine serving as anetcher is absorbed in a hole where a silicon atom used to exist. Thus,etching of the film is developed.

Hereinbelow are explained embodiments in which gases other than N₂ andSiH₄ used in the above mentioned embodiments are used to deposit afluorinated amorphous carbon film containing nitrogen and silicon. Anamorphous carbon film containing nitrogen was deposited using CF₄ gas ora mixture gas of CF₄ and CH₄ to which NO, NO₂, NH₃ or NF₃ gas was addedas nitrogen source. The thus formed amorphous carbon film containingnitrogen has the same heat resistance and etching property as theamorphous carbon film deposited using N₂ gas.

Various process gases may be used to deposit an amorphous carbon filmcontaining nitrogen or silicon. For instance, amorphous carbon filmswere deposited using C₂ F₆, C₃ F₈, C₄ F₈ or CHF₃ gas in place of CF₄, towhich gas H₂, C₂ H₆, C₂ H₄, C₂ H₂ or C₃ H₈ was added as hydrogen source,and to each of which N₂, NO, NO₂, NH₃ or NF₃ was further added asnitrogen source. Each of the thus deposited amorphous carbon filmsexhibits the same heat resistance and etching property.

As to an amorphous carbon film containing silicon therein, an amorphouscarbon film was deposited using CF₄ gas or a mixture gas of CF₄ and CH₄to which Si₂ H₆ or SiF₄ gas was added as silicon source. The thus formedamorphous carbon film containing silicon has the same heat resistanceand etching property as the amorphous carbon film deposited using SiH₄gas. There may be used gases other than CF₄ and CH₄ as process gases.For instance, amorphous carbon films were deposited using C₂ F₆, C₃ F₈,C₄ F₈ or CHF₃ gas in place of CF₄, to which gas H₂, C₂ H₆, C₂ H₄, C₂ H₂or C₃ H₈ was added as hydrogen source, and to each of which SiH₄, Si₂ H₆or SiF₄ was further added as silicon source. Each of the thus depositedamorphous carbon films exhibits the same heat resistance and etchingproperty. Since the method for depositing the amorphous carbon film usesplasma, any gas may be used if it contains nitrogen or silicon. Inaddition, there may be used highly densified plasma caused by microwavedischarge or helicon wave discharge. Either of them can provide the sameadvantageous effects as those obtained by high frequency discharge.

There was fabricated a MOSFET semiconductor device having a structure asillustrated in FIG. 3 in which the interlayer insulative material 34 iscomposed of the fluorinated amorphous carbon film containing nitrogen orsilicon. A semiconductor device having an interlayer insulative layercomposed of an amorphous carbon film can have heat resistance against420 degrees centigrade at maximum due to gas generation out of theamorphous carbon film. On the other hand, the amorphous carbon filmincluding an interlayer insulative layer composed of the fluorinatedamorphous carbon film containing nitrogen or silicon can withstand heattreatment at 470 degrees centigrade with the result that contactresistance in wirings is reduced. Thus, signal transmission speed inwirings can be made higher by about 5% relative to that of asemiconductor device having an interlayer insulative layer composed ofan amorphous carbon film containing no nitrogen and silicon. Inaddition, since the same gas and resist as those to be used inconventional SiO₂ etching can be used in etching for patterning, andfurther since resist removal can be carried out by conventional oxygenplasma, a semiconductor device having an interlayer insulative layercomposed of the amorphous carbon film containing nitrogen and siliconcan be fabricated through the same pattering steps as those to be usedfor fabricating a semiconductor device including an interlayerinsulative layer composed of SiO₂. The amorphous carbon film containingnitrogen or silicon can be applied to a semiconductor device constructedof a bipolar transistor as well as a semiconductor device constructed ofMOSFET illustrated in FIG. 3.

Hereinbelow will be described embodiments of a semiconductor deviceincluding a buffer layer in accordance with the invention. FIG. 17Aillustrates a semiconductor device constructed of an npn type bipolartransistor in accordance with an embodiment of the present invention.

The bipolar transistor comprises a p type semiconductor substrate 40 onwhich an n⁺ diffusion layer 41 is formed. Over the n⁺ diffusion layer 41is formed an epitaxial n type layer 42, and beside the n type layer 42is formed a p⁺ isolation layer 43 by ion implantation. On the epitaxialn type layer 42 is formed a p type layer 44 by ion implantation. The ptype layer 44 serves as a base. Between the epitaxial n type layer 42and the p type layer 44 is formed an n⁺ emitter layer 45. On theepitaxial n type layer 42 is formed an n⁺ type polysilicon electrode 46which is connected with the n⁺ type diffusion layer 41 through an n⁺type layer 47. On the n⁺ type emitter layer 45 is also formed the n⁺type polysilicon electrode 46. On the p type layer 44 and the n⁺ typepolysilicon electrodes 46 are formed metal electrodes 48 serving as agate.

In the semiconductor device illustrated in FIG. 17A, an amorphous carbonfilm 49 is arranged not to come to direct contact with active regions ofa transistor and wirings. Namely, a buffer layer 50 composed of SiO₂ isformed between the amorphous carbon film 49 and the active regions andwirings. The buffer layer 12 is deposited as follows.

After defining active regions of a transistor, polysilicon and metal aredeposited over the substrate. Then, the deposited polysilicon and metalare patterned in a conventional manner. Then, a thin SiO₂ layer isdeposited over a transistor region by plasma-enhanced chemical vapordeposition (PCVD). In this embodiment, a thin SiO₂ layer having athickness of 0.01 μm is used as the buffer layer 50. Then, over thebuffer layer 50 composed of a thin SiO₂ layer is deposited the amorphouscarbon film 49 by about 1 μm thickness as an interlayer insulativelayer. Further, over the amorphous carbon film 49 are deposited aluminumwirings 51. The aluminum wirings 51 are also covered with the thin SiO₂buffer layer 50 so that the aluminum wirings 51 do not come to directcontact with the amorphous carbon film 49.

The reason why the SiO₂ buffer layer 50 has a thickness of 0.01 μm isbased on the discovery that if the SiO₂ buffer layer 50 has a thicknesssmaller than 0.01 μm, the SiO₂ layer does not work as a buffer layeragainst gas discharged out of the amorphous carbon film 49 when the film49 is under heat treatment at 500 degrees centigrade. On the other hand,if the buffer layer 50 has a thickness greater than 0.01 μm, a totaldielectric constant of the amorphous carbon film 49 is increased. Forthis reason, the buffer layer 50 is preferably as thin as possible.

Accordingly, a thickness of the buffer layer 50 is determined by atemperature of heat treatment to be carried out in semiconductor devicefabrication process. If a semiconductor device is allowed to have lowheat resistance, the buffer layer 50 may have a thickness smaller than0.01 μm. Even if the buffer layer 50 has such a thickness, the bufferlayer 50 serves as a buffer layer against gas discharged out of theamorphous carbon film. On the other hand, if a semiconductor device isrequired to have high heat resistance, it is necessary for the bufferlayer 50 to have a thickness greater than 0.01 μm. In this embodiment,the SiO₂ layer having a dielectric constant of 4 is deposited by 0.01μm, while the amorphous carbon film 49 having a dielectric constant of2.3 is deposited by 1 μm. A dielectric constant of the SiO₂ layer andthe amorphous carbon film 49 as a whole is 2.3 provided that a capacitoris in connection with each of the layer and film in series. Namely, theincrease of a dielectric constant caused by the SiO₂ layer can bedisregarded.

Hereinbelow is explained a second embodiment with reference to FIG. 17B.In this embodiment, a semiconductor device is constructed of n channeltype MOSFET. The semiconductor device has a p type semiconductorsubstrate 40 on which field SiO₂ films 52 are formed except areas whichwould be used as active regions of a semiconductor device. In the activeregions are formed a source 53 and a drain 54 by ion implantation.Centrally between the source 53 and the drain 54 is formed a gateelectrode 55 on a thin SiO₂ film (not illustrated), which gate electrode55 is composed of polysilicon. Over these contacts is deposited thebuffer layer 50 composed of a thin SiO₂ layer, and over the thin SiO₂buffer layer 50 is deposited the amorphous carbon film 49. In thisembodiment, the SiO₂ buffer layer 50 has a thickness of 0.01 μm,similarly to the previously mentioned embodiment.

Many variations of the SiO₂ buffer layer 50 are deposited for comparisonwith the buffer layer 50 as follow.

Variation 1: An Si₃ N₄ buffer layer is deposited by 0.01 μm thickness inplace of the SiO₂ layer in the first embodiment (FIG. 17A).

Variation 2: An amorphous carbon film containing silicon by 40% isdeposited as a buffer layer in place of the SiO₂ layer in the firstembodiment.

Variation 3: An amorphous carbon film containing nitrogen by 40% isdeposited as a buffer layer in place of the SiO₂ layer in the firstembodiment.

Variation 4: An Si₃ N₄ buffer layer is deposited by 0.01 μm thickness inplace of the SiO₂ layer in the second embodiment (FIG. 17A).

Variation 5: An amorphous carbon film containing silicon by 40% isdeposited as a buffer layer in place of the SiO₂ layer in the secondembodiment.

Variation 6: An amorphous carbon film containing nitrogen by 40% isdeposited as a buffer layer in place of the SiO₂ layer in the secondembodiment.

These variations were heated up to 600 degrees centigrade for failuretest of wirings. The results are shown in the following table. Thetemperature listed in the table indicates a temperature at which thereoccurs defectiveness such as irregularity caused by that gas broken outof the amorphous carbon film due to decomposition of the film blows offto the wirings.

    ______________________________________                                        Semiconductor Devices in FIGS. 1A and 1B                                                              420° C.                                        First Embodiment (FIG. 17A)                                                                                         500° C.                          Second Embodiment (FIG. 17B)                                                                                       500° C.                           Variation 1                                           520° C.          Variation 2                                           470° C.          Variation 3                                           470° C.          Variation 4                                           520° C.          Variation 5                                           470° C.          Variation 6                    470° C.                                 ______________________________________                                    

It has been found in a prior semiconductor device that a thresholdvoltage of MOSFET was varied by heat treatment at 500 degreescentigrade. However, it Is possible in the second embodiment (FIG. 17B),in which a transistor section is covered with the SiO₂ buffer layer, toprevent a threshold voltage from varying due to heat treatment thereofat 500 degrees centigrade. The reason why the threshold voltage isvaried in prior semiconductor device is considered that impuritiesderived from gas broken out of the amorphous carbon film while heattreatment enter a gate oxide layer of a transistor.

In the first embodiment illustrated in FIG. 17A, as illustrated in FIG.18B, the structure suddenly changes from the amorphous carbon film tothe SiO₂ layer at the interface of the amorphous carbon film and theSiO₂ layer. In an embodiment described hereinbelow, the structurethereof is not suddenly changed at the interface, but gradually changedfrom the amorphous carbon film to the SiO₂ layer, as illustrated in FIG.18A. This embodiment has a transition layer having a thickness of about50 angstroms, in which transition layer carbon and fluorine contents aregradually decreased, while silicon and oxygen contents are graduallyincreased. Thus, the transition layer is composed of the amorphouscarbon film at one end, but composed of the SiO₂ layer at opposite end.Over the transition layer is deposited SiO₂ by 50 angstroms to therebyconstruct the buffer layer of a combination of the transition layer andthe SiO₂ layer.

A semiconductor device in accordance with this embodiment was testedwith respect to heat resistance. The same heat resistance could beobtained as that of the first embodiment (FIG. 17A) in which thestructure is suddenly changed across the interface. The inventorincorporated the 50 μm thick transition layer into semiconductor devicesin accordance with the second embodiment and the variations 1 to 6, andtested them with respect to heat resistance. The same heat resistancecould be obtained as that of the first embodiment in which the structureis suddenly changed across the interface.

The buffer layers used in the above mentioned embodiments and variationshave different etching rates from those of the amorphous carbon film,fluorinated amorphous carbon film and resist material in CF₄ or oxygenplasma etching. Thus, the buffer layers can work as an etching stopperlayer in an amorphous carbon film etching step which is indispensablefor fabricating a semiconductor device in accordance with the invention,or in resist ashing step which is to be carried out after etching theamorphous carbon film with resist.

FIG. 19 illustrates an apparatus for carrying out the method inaccordance with the invention. The illustrated apparatuses is providedwith helicon wave plasma source. The apparatus has a vacuum chamber 40which is in communication with and is to be made vacuous by a vacuumpump 41. A part of the vacuum chamber 40 defines a plasma chamber 42surrounded with a plurality of magnets 43. A voltage supply 44 provideshigh frequency electric power with plasma source in the plasma chamber42. Within the vacuum chamber 40 is disposed a support plate 45 on whicha specimen 46 such as a semiconductor substrate is placed just below theplasma chamber 42. It should be noted that the specimen 46 is notdisposed in the plasma chamber 42, but disposed outside the plasmachamber 42. A voltage supply 47 is in communication with the supportplate 45, and thus supplies high frequency voltage with the supportplate 45. As a result, a bias voltage can be applied to the specimen 46through the support plate 45 from the voltage supply 47. The supportplate 45 is also in communication with a temperature controller 48, andthus is able to be heated or cooled to a desired temperature. A gascontainer 49 is in fluid communication with the vacuum chamber 40, andthus supplies carbon fluoride gas such as CF₄, C₂ F₆, C₃ F₈ and C₄ F₈into the vacuum chamber 40.

In operation, the specimen 46 such as a silicon substrate is placed onthe support plate 45, and then carbon fluoride gas is introduced intothe vacuum chamber 40 from the gas container 49. Then, the voltagesupply 44 applies high frequency voltage for discharge to the plasmasource at 10⁻³ Torr to thereby generate carbon fluoride plasma. Anamorphous carbon film is deposited on the specimen 46 by the thusgenerated carbon fluoride plasma.

FIG. 20 illustrates another apparatus for carrying out the method inaccordance with the invention. The illustrated apparatuses is providedwith microwave plasma source. The apparatus has a vacuum chamber 50which is in communication with and is to be made vacuous by a vacuumpump 51. A part of the vacuum chamber 50 defines a plasma chamber 52surrounded with a plurality of magnets 53. A voltage supply 54 providesmicrowave electric power with plasma source in the plasma chamber 52.Within the vacuum chamber 50 is disposed a support plate 55 on which aspecimen 56 such as a semiconductor substrate is placed just below theplasma chamber 52. It should be noted that the specimen 56 is notdisposed in the plasma chamber 52, but disposed outside the plasmachamber 52. A voltage supply 57 is in communication with the supportplate 55, and thus supplies high frequency voltage with the supportplate 55. As a result, a bias voltage can be applied to the specimen 56through the support plate 55 from the voltage supply 57. The supportplate 55 is also in communication with a temperature controller 58, andthus is able to be heated or cooled to a desired temperature. A gascontainer 59 is in fluid communication with the vacuum chamber 50, andthus supplies carbon fluoride gas such as CF₄, C₂ F₆, C₃ F₈ and C₄ F₈into the vacuum chamber 50.

The apparatus operates in the same way as the apparatus illustrated inFIG. 19.

In a conventional parallel flat plate type plasma generator, anamorphous carbon film is deposited on condition that total gas flow rateis kept to be 50 sccm, and high frequency electric power (13.56 MHz) of200 W is applied to a mixture gas of CF₄ and CH₄. FIG. 21 shows arelationship between deposition speed and gas flow rate, and FIG. 22shows a relationship between a dielectric constant of an amorphouscarbon film and gas flow rate both in a conventional apparatus. Thedeposition speed and dielectric constant (1 MHz) of an amorphous carbonfilm vary in dependence on mixture rate of plasma source, and thus varyas shown in FIGS. 13 and 14. It is possible to obtain an amorphouscarbon film having a dielectric constant smaller than 3 even by aconventional apparatus, however, the conventional apparatus providesonly low deposition speed, which poses a problem of a small throughput.

Since it is considered that the use of highly densified plasma wouldincrease an amount of radicals contributing to deposition with theresult of higher deposition speed, the inventor had deposited anamorphous carbon film through the use of highly densified plasmagenerated by helicon waves. There was used 100 sccm of CF₄ gas and C₂ F₆gas, respectively, as plasma source which was diluted with hydrogen gas(H₂). High frequency electric power (13.56 MHz) used for producinghelicon waves was fixed at 2 kW. The temperature of the support platewas cooled down to 50 degrees centigrade.

FIG. 23 shows the dependency of deposition speed on H₂ flow rate. Thecurve 1 indicates the dependency when CF₄ gas was used, while the curve2 indicates the dependency when C₂ F₆ gas was used. As results ofmeasurement, the electron density was found to be 5×10¹² cm⁻³, andplasma potential was found to be 20 V. As will be understood from FIG.23, the use of helicon plasma makes it possible to deposit a fluorinatedamorphous carbon film without addition of hydrogen. That is, thedeposition speeds of about 150 nm/min (curve 1) and about 300 nm/min(curve 2) were obtained when H₂ flow rate is zero. (As earliermentioned, in a conventional apparatus, when H₂ flow rate was zero, thedeposition speed was also zero.)

Namely, the inventor has found that the use of highly densified plasmain which a substrate is disposed separately from plasma generatingregion. makes it possible to deposit a fluorinated amorphous carbon filmconsisting of carbon and fluorine atoms. In addition, the inventor hasalso established the method by which deposition speed of an amorphouscarbon film can be increased about ten times greater than depositionspeed obtained by a conventional parallel flat plate type apparatus.

The inventor had also deposited an amorphous carbon film through the useof highly densified plasma generated by microwaves. There was used 100sccm of CF₄ gas and C₂ F₆ gas, respectively, as plasma source which wasdiluted with hydrogen gas (H₂). Microwave electric power (2.45 GHz) usedfor producing microwaves was kept to be 2 kW. The temperature of thesupport plate was cooled down to 50 degrees centigrade.

FIG. 24 shows the dependency of deposition speed on H₂ flow rate. Thecurve 1 indicates the dependency when CF₄ gas was used, while the curve2 indicates the dependency when C₂ F₆ gas was used. As results ofmeasurement, the electron density was found to be 2×10¹² cm⁻³, andplasma potential was found to be 16 V. As will be understood from FIG.24, the use of microwave plasma makes it possible to deposit afluorinated amorphous carbon film without addition of hydrogen. That is,the deposition speeds of about 100 nm/min (curve 1) and about 280 nm/min(curve 2) were obtained when H₂ flow rate is zero. (As earliermentioned, in a conventional apparatus, when H₂ flow rate was zero, thedeposition speed was also zero.) The deposition speed obtained when H₂flow rate is zero is smaller than the deposition speed obtained by theabove mentioned helicon plasma, but is much greater than the depositionspeed obtained by a conventional parallel flat plate type apparatus.

The reason why the use of highly densified plasma can remarkably enhancethe deposition speed relative to that of the conventional parallel flatplate type apparatus is considered that density of radicals whichcontribute to deposition of the film is increased relative to theconventional parallel flat plate type apparatus. The reason why afluorinated amorphous carbon film can be deposited without dilution withhydrogen is considered that these highly densified plasma sources areaccelerated in accordance with a difference in potential between thesubstrate and the plasma, and hence energy of ions irradiated to theamorphous carbon film can be made smaller than that of the conventionalparallel flat plate type apparatus with the result that etching can besuppressed.

In the above mentioned embodiments, the process gases CF₄ and C₂ F₆ areused for deposition of the amorphous carbon film. However, it should benoted that the process gas is not limited to those, and that othercarbon fluoride gases such as C₃ F₈ and C₄ F₈ may be used. In theexperiments which had been carried out by the inventor, when carbonfluoride gas such as C₃ F₈ and C₄ F was used, the conventional parallelflat plate type apparatus could not deposit an amorphous carbon film. Onthe other hand, the use of highly densified plasma made it possible todeposit an amorphous carbon film with carbon fluoride gas such as C₃ F₈and C₄ F. In addition, the deposition speed was the same as thatobtained when C₂ F₆ gas was used.

The inventor had also investigated how quality of the amorphous carbonfilm is influenced by varying energy of ions irradiated to a specimen.The specimen was fabricated without hydrogen gas, but only with CF₄ andC₂ F₆ gases. The ion energy was varied by applying high frequencyvoltage (400 KHz) to the support plate to thereby control a voltage ofthe specimen. A temperature of the support plate was kept to be 50degrees centigrade. In a conventional parallel flat plate typeapparatus, since a self-bias voltage is applied to a specimen placed onan electrode, it was difficult to control ion energy by controlling abias voltage. However, the use of highly densified plasma such ashelicon wave and microwave and the arrangement of deposition area beingseparately disposed from plasma generation area make it possible tocontrol a voltage of the support plate by applying high frequencyelectric power to the support plate, and thus also possible to controlenergy of ions irradiated to a substrate.

FIG. 25 shows how the deposition speed of the amorphous carbon filmdeposited using CF₄ gas varies as the high frequency bias electric powervaries. The curve 1 indicates the deposition speed of the film depositedusing helicon wave, while the curve 2 indicates the deposition speed ofthe film deposited using microwave. It has been found that as biasvoltage increases, the deposition speeds decrease, and that an amorphouscarbon film can not be deposited if bias electric power is over 200 W.The reason of this phenomenon is considered that since ion energyincreases as the bias electric power increases, etching is facilitated.The reason why an amorphous carbon film was not deposited only with CFfamily gas in a conventional parallel flat plate type apparatus isconsidered that there exist ions having energy corresponding to energyobtained when bias electric power of 200 W or greater is applied by thehighly densified plasma source. It was also observed that while anamorphous carbon film is being deposited with carbon fluoride gases suchas C₃ F₈ and C₄ F₈ by using the highly densified plasma, the depositionspeed of the film is decreased by applying a bias voltage to thespecimen.

How heat resistance, dielectric constant and pattern-fillingcharacteristic of the amorphous carbon film were influenced by biaselectric power application was tested. Specifically, the followings weremeasured: a temperature at which the amorphous carbon film begins to bedecomposed, and thus weight of the film begins to decrease when the filmis heated in vacuum atmosphere, fluorine content in the film, and adielectric constant (1 MHz) of the film. The results are shown in thefollowing table.

    ______________________________________                                        Plas-               Bias             D. Speed                                                                             H. R.                             ma        Gas       [W]    D. C.                                                                              F [%]                                                                              [μm/min]                                                                          [° C.]                     ______________________________________                                        Reference                                                                            F      CH.sub.4 + CF.sub.4                                                                     --   2.1  54   0.035  280                             Sample 1                                                                               H         CF.sub.4                                                                                   2.3                                                                                55                                                                                 0.3      300                        Sample 2                                                                               H         CF.sub.4                                                                                 2.4    52                                                                                 0.15                                                                                  380                         Sample 3                                                                               H         CF.sub.4                                                                                 2.5    51                                                                                 0.13                                                                                  400                         Sample 4                                                                               H         C.sub.2 F.sub.6                                                                        0                                                                                 2.4                                                                                53                                                                                 0.10                                                                                  330                         Sample 5                                                                               H         C.sub.2 F.sub.6                                                                        100                                                                             2.5    50                                                                                 0.28                                                                                  410                         Sample 6                                                                               H         C.sub.2 F.sub.6                                                                        150                                                                             2.6    48                                                                                 0.25                                                                                  470                         Sample 7                                                                               M         CF.sub.4                                                                                   2.2                                                                                58                                                                                 0.1      280                        Sample 8                                                                               M         CF.sub.4                                                                                 2.30                                                                                 56                                                                                 0.08                                                                                  300                         Sample 9                                                                               M         CF.sub.4                                                                                 2.40                                                                                 53                                                                                 0.05                                                                                  340                         Sample 10                                                                             M          C.sub.2 F.sub.6                                                                         0                                                                                2.3                                                                                55                                                                                 0.28                                                                                  300                         Sample 11                                                                             M          C.sub.2 F.sub.6                                                                         100                                                                            2.4    52                                                                                 0.26                                                                                  380                         Sample 12                                                                             M          C.sub.2 F.sub.6                                                                         150                                                                            2.5    50                                                                                 0.23                                                                                  410                         ______________________________________                                    

In the above table, F, H and M means parallel flat plate type plasma,helicon wave plasma, and microwave plasma, respectively. D.C. means adielectric constant, F means fluorine (F) content, D. Speed meansdeposition speed of an amorphous carbon film, and H.R. means heatresistance of an amorphous carbon film.

As is understood from the above table, it has been found that theapplication of bias voltage enhances the heat resistance of theamorphous carbon film. That is, the inventor has found that the use ofhighly densified plasma and application of high frequency electric powerto a specimen during the deposition are quite helpful for enhancement ofthe heat resistance of the fluorinated amorphous carbon film. It hasalso been found that fluorine content in the film is decreased byapplying bias electric power for increasing irradiation energy. Thedecrease of fluorine content in the film facilitates formation ofcarbon-carbon bonding in the film, which is considered to contribute toincreasing cross-linking degree of the film. The heat resistance of afilm is in general dependent on cross-linking degree of a structure ofthe film. Accordingly, enhancement of the heat resistance by applicationof a bias voltage is considered due to increased cross-linking degree ofthe film. It is considered that in a conventional parallel flat platetype apparatus listed as a reference case in the above table, to a filmare irradiated ions having energy corresponding to energy obtained whena bias voltage is applied with highly densified plasma. The reason whythe film in a reference case has low heat resistance regardless of suchirradiation of ions thereto is considered that hydrogen gas has to beused in deposition of the film, and hence some hydrogen atoms aredesorbed in the film at lower temperature.

The fluorinated amorphous carbon film was deposited on a siliconsubstrate on which aluminum wirings had been already patterned inconventional manner. Then, how degree the fluorinated amorphous carbonfilm can be filled in spacings among the patterned wirings was observed.The deposition of the film was carried out on condition that wiringsconstituting a pattern have a width of 0.4 μm, a spacing betweenadjacent wirings is 0.4 mm, and a height of wirings is 0.8 μm. When anamorphous carbon film was deposited through the use of helicon wave ormicrowave without application of a bias voltage to a substrate, even ifany gas among CF₄, C₂ F₆, C₃ F₈ and C₄ F₈ was used, the pattern was notable to be filled with the amorphous carbon film, and hence some voidswere found among wirings. On the other hand, when the deposition of anamorphous carbon film was carried out with a bias voltage applied to thesubstrate, the pattern was sufficiently filled with the fluorinatedamorphous carbon film without occurrence of voids. In general, voids aregenerated when an amorphous carbon film has smaller deposition speed todeposit on a side wall of material such as aluminum for formation ofwirings than deposition speed to deposit on a top surface of thematerial. The reason why application of a bias electric power is helpfulfor filling a pattern with an amorphous carbon film is considered thatthe application of a bias electric power accelerates ions and thusselectively facilitates only etching which occurs on the wiring materialwith the result of reduction of a difference between the depositionspeed of the film to deposit on a side wall of wiring material and thedeposition speed of the film to deposit on a top surface of the wiringmaterial.

Though helicon wave and microwave are used in the above mentionedembodiments, any plasma source may be used if that plasma is highlydensified, and further if a substrate on which an amorphous carbon filmis to deposit is disposed separately from plasma generation area. Forinstance, inductive coupling type plasma may be selected.

While the present invention has been described in connection withcertain preferred embodiments, it is to be understood that the subjectmatter encompassed by way of the present invention is not to be limitedto those specific embodiments. On the contrary, it is intended for thesubject matter of the invention to include all alternatives,modifications and equivalents as can be included within the spirit andscope of the following claims.

What is claimed is:
 1. A method of fabricating a semiconductor device,comprising the step of forming an interlayer insulative layer composedof an amorphous carbon film including fluorine (F), where aconcentration of fluorine in the amorphous carbon film varies in adepth-wise direction to enhance cohesion of the film with a substrate,by plasma-enhanced chemical vapor deposition (PCVD) using a mixture gasincluding (a) at least one of CF₄, C₂ F₆, C₃ F₈, and CHF₃, and (b) atleast one of N₂, NO, NO₂, NH₃, and NF₃.
 2. The method as recited inclaim 1, wherein said mixture gas further including (c) at least one ofH₂, CH₄, C₂ H₆, C₂ H₄, C₂ H₂, and C₃ H₈.
 3. The method of fabricating asemiconductor device as recited in claim 1, wherein said interlayerinsulative layer composed of amorphous carbon film including fluorine(F) is formed to have a content of said fluorine that decreases in saiddepth-wise direction of said amorphous carbon film.
 4. The method asrecited in claim 3, further comprising the step of forming a bufferlayer for suppressing gas discharge out of said amorphous carbon film,said buffer layer being disposed between said amorphous carbon film andelements of said semiconductor device disposed adjacent to saidamorphous carbon film.
 5. The method of fabricating a semiconductordevice as recited in claim 4, further comprise the steps of:providing asemiconductor substrate; and forming a transistor on said semiconductorsubstrate over which said interlayer insulative layer of an amorphouscarbon film is formed.
 6. A method of fabricating a semiconductordevice, comprising the step of forming an interlayer insulative layercomposed of an amorphous carbon film including fluorine (F), where aconcentration of fluorine in the amorphous carbon film varies in adepth-wise direction to enhance cohesion of the film with a substrate,by plasma-enhanced chemical vapor deposition (PCVD) using a mixture gasincluding (a) at least one of CF₄, C₂ F₆, C₃ F₈, and CHF₃, and (b) atleast one of SiH₄, Si₂ H₆, and SiF₄.
 7. The method as recited in claim6, wherein said mixture gas further including (c) at least one of H₂,CH₄, C₂ H₆, C₂ H₄, C₂ H₂, and C₃ H₈.
 8. The method of fabricating asemiconductor device as recited in claim 6, wherein said interlayerinsulative layer composed of amorphous carbon film including fluorine(F) is formed to have a content of said fluorine that decreases in saiddepth-wise direction of said amorphous carbon film.
 9. The method asrecited in claim 8, further comprising the step of forming a bufferlayer for suppressing gas discharge out of said amorphous carbon film,said buffer layer being disposed between said amorphous carbon film andelements of said semiconductor device disposed adjacent to saidamorphous carbon film.
 10. The method of fabricating a semiconductordevice as recited in claim 9, further comprising the steps of:providinga semiconductor substrate; and forming a transistor on saidsemiconductor substrate over which said interlayer insulative layer ofan amorphous carbon film is formed.
 11. A method of fabricating asemiconductor device, comprising the step of forming an interlayerinsulative layer composed of an amorphous carbon film including fluorine(F) having a content that decreases in a depth-wise direction of saidamorphous carbon film, formed by carrying out plasma-enhanced chemicalvapor deposition (PCVD) using CxFy gas, wherein x is an integer rangingfrom 1 to 4 both inclusive and y is an integer ranging from 4 to 8 bothinclusive.
 12. The method as recited in claim 11, wherein said plasma isgenerated by microwave discharge.
 13. The method as recited in claim 11,wherein said plasma is generated by helicon wave discharge.
 14. Themethod of fabricating a semiconductor device as recited in claim 11,further comprising the step of forming a buffer layer for suppressinggas discharge out of said amorphous carbon film, said buffer layer beingdisposed between said amorphous carbon film and elements of saidsemiconductor device disposed adjacent to said amorphous carbon film.15. The method of fabricating a semiconductor device as recited in claim14, further comprising the steps of:providing a semiconductor substrate;and forming a transistor on said semiconductor substrate over which saidinterlayer insulative layer of an amorphous carbon film is formed. 16.The method of fabricating a semiconductor device as recited in claim 15,wherein said semiconductor substrate on which said amorphous carbon filmto be deposited is disposed outside an area in which plasma isgenerated.
 17. A method of fabricating a semiconductor device,comprising the step of forming an interlayer insulative layer composedof an amorphous carbon film including fluorine (F) having a content thatdecreases in a depth-wise direction of said amorphous carbon film,formed by carrying out plasma-enhanced chemical vapor deposition (PCVD)using CxFy gas, wherein x is an integer ranting from 1 to 4 bothinclusive, and y is an integer ranting from 4 to 8 both inclusive, andproviding a frequency of electric power with said substrate while PCVDis being carried out.
 18. The method as recited in claim 17, whereinsaid plasma is generated by microwave discharge.
 19. The method asrecited in claim 17, wherein said plasma is generated by helicon wavedischarge.
 20. The method of fabricating a semiconductor device asrecited in claim 17, further comprising the step of forming a bufferlayer for suppressing gas discharge out of said amorphous carbon film,said buffer layer being disposed between said amorphous carbon film andelements of said semiconductor device disposed adjacent to saidamorphous carbon film.
 21. The method of fabricating a semiconductordevice as recited in claim 20, further comprising the steps of:providinga semiconductor substrate; and forming a transistor on saidsemiconductor substrate over which said interlayer insulative layer ofan amorphous carbon film is formed.
 22. The method of fabricating asemiconductor device as recited in claim 21, wherein said semiconductorsubstrate on which said amorphous carbon film is to be deposited isdisposed outside an area in which plasma is generated.
 23. A method offabricating a semiconductor device, comprising the steps of:forming aninterlayer insulative layer composed of amorphous carbon film includingfluorine (F) so that a content of said fluorine decreases in adepth-wise direction of said amorphous carbon film.
 24. The method offabricating a semiconductor device as recited in claim 23, furthercomprising the step of forming a buffer layer for suppressing gasdischarge out of said amorphous carbon film, said buffer layer beingdisposed between said amorphous carbon film and elements of saidsemiconductor device disposed adjacent to said amorphous carbon film.25. The method of fabricating a semiconductor device as recited in claim23, wherein said interlayer insulative layers further includes nitrogen(N).
 26. The method of fabricating a semiconductor device as recited inclaim 23, wherein said interlayer insulative layer further includessilicon (Si).
 27. A method of fabricating a semiconductor device,comprising the steps of:forming an interlayer insulative layer composedof amorphous carbon film; forming a buffer layer for suppressing gasdischarge out of said amorphous carbon film, said buffer layer beingdisposed between said amorphous carbon film and elements of saidsemiconductor device disposed adjacent to said amorphous carbon film;and forming a transition layer interposed between said amorphous carbonfilm and said buffer layer, said transition layer having a compositiongradually varying from a composition of said amorphous carbon film to acomposition of said buffer layer.
 28. The method of fabricating asemiconductor device as recited in claim 27, wherein said interlayerinsulating layer includes fluorine (F).
 29. The method of fabricating asemiconductor device as recited in claim 28, wherein said fluorine (F)decreases in a depth-wise direction of said interlayer insulating layer.30. The method of fabricating a semiconductor device as recited in claim29, wherein none of said fluorine (F) is present at an interface betweensaid interlayer insulating layer and an underlying layer disposed belowsaid interlayer insulating layer.
 31. The method of fabricating asemiconductor device as recited in claim 30, wherein said underlyinglayer is a semiconductor substrate.
 32. A method of fabricating asemiconductor device, comprising the step of:forming an interlayerinsulative layer composed of an amorphous carbon film including fluorine(F), where a concentration of fluorine in the amorphous carbon filmvaries in a depth-wise direction to enhance cohesion of the film with asubstrate; and forming a buffer layer for suppressing gas discharge outof said amorphous carbon film, said buffer layer being disposed betweensaid amorphous carbon film and elements of said semiconductor devicedisposed adjacent to said amorphous carbon film.
 33. The method offabricating a semiconductor device recited in claim 32, furthercomprising the step of forming a transition layer interposed betweensaid amorphous carbon film and said buffer layer.
 34. The method offabricating a semiconductor device recited in claim 33, wherein saidinterlayer insulating layer of an amorphous carbon film includesfluorine (F).
 35. The method of fabricating a semiconductor device asrecited in claim 34, wherein said fluorine (F) decreases in saiddepth-wise direction of said interlayer insulating layer.
 36. The methodof fabricating a semiconductor device as recited in claim 35, whereinnone of said fluorine (F) is present at an interface between saidinterlayer insulating layer and an underlying layer disposed below saidinterlayer insulating layer.